1. Field of the Invention
The present invention relates to lithium secondary batteries, and more particularly a lithium secondary battery having an electrode assembly provided with a curved portion.
2. Description of Related Art
Various mobile communication devices and mobile electronic devices such as laptop computers have emerged in recent years, and this has lead to a demand for higher capacity batteries as their driving power sources. Lithium secondary batteries, which perform charge and discharge by transferring lithium ions between the positive and negative electrodes, have been widely used as the driving power source for the mobile communication devices and the like since they have higher energy density and greater high capacity than other secondary batteries such as nickel-cadmium storage batteries. Nevertheless, as size and weight reductions in the mobile communication devices and other electronic devices advance, the demand for further improvements in energy density and cycle performance of lithium secondary batteries is expected to increase.
Currently, in common lithium secondary batteries, carbon materials represented by graphite are generally used as their negative electrode materials (negative electrode active materials). When using a negative electrode material composed of graphite, lithium intercalation is only possible up to the composition LiC6, and the upper limit of battery capacity is limited to the theoretical capacity 372 mAh/g. This has been an obstacle to improvements in battery capacity.
In view of this problem, a lithium secondary battery employing aluminum, silicon, or tin that alloys with lithium as a negative electrode active material with a high energy density per weight and per volume is reported in Solid State Ionics, Vols. 113-115, p. 57 (1998). Among the materials, silicon shows a particularly high theoretical capacity and is therefore desirable as a negative electrode active material for high-capacity batteries, so various secondary batteries using silicon as negative electrode active material have been proposed (see, for example, Japanese Published Unexamined Patent Application No. 10-255768).
A problem with the negative electrode that uses this type of active material, however, has been that it undergoes considerable changes in volume during the charge-discharge process, and thereby stress develops between the negative electrode active material and the negative electrode current collector, eventually resulting in peel-off of the negative electrode active material and creases or warpage of the electrode.
In view of the problem, Japanese Published Unexamined Patent Application No. 2001-266851, for example, proposes a negative electrode for a lithium secondary battery has been proposed in which a microcrystalline thin film or an amorphous thin film of silicon or the like is formed on a negative electrode current collector made of, for example, a copper foil by a thin-film forming technique such as evaporation or sputtering. In the negative electrode of this type, the microcrystalline or amorphous thin film on the negative electrode current collector is provided with columnar structures, which serves to alleviate the stress due to the expansion and shrinkage of the negative electrode active material associated with the charge-discharge process and prevents the negative electrode active material from peeling off from the negative electrode current collector. Therefore, the charge-discharge cycle performance of the lithium secondary battery improves to a certain extent, but the improvement effect is still insufficient.
Japanese Published Unexamined Patent Application No. 2001-273892, for example, also proposes a lithium secondary battery that employs an electrode material made of, for example, a silicon alloy containing an active material that intercalates and deintercalates lithium, such as silicon, and an additive metal that does not intercalate or deintercalate lithium. The electrode for a lithium secondary battery of this type can alleviate the expansion and shrinkage associated with the charge-discharge process to a certain extent and can consequently reduce the stress due to because of the presence of the metal that does not intercalate or deintercalate lithium. Moreover, due to the presence of the metal that does not intercalate or deintercalate lithium, structural changes in the active material film associated with the charge-discharge process can be alleviated, and the peel-off of the active material due to the stress can be lessened to some extent.
Nevertheless, even the lithium secondary batteries fabricated through these manufacturing methods cannot reduce the stress associated with charge and discharge sufficiently, so that when the charge-discharge cycle is repeated over a long period of time, they still suffer from capacity degradation resulting from deformation of the negative electrode active material that occurs at early stage of charge-discharge cycling and creases and warpage of the negative electrode due to non-uniform plastic deformation of the negative electrode, which lead to the problem of cycle performance deterioration.